Enhanced fuel delivery for direct methanol fuel cells

ABSTRACT

An arrangement for a direct methanol fuel cell includes a fuel cartridge that supplies a source of fuel to the direct methanol fuel cell. The fuel cartridge has a surface area enhanced planar vaporization membrane residing in the fuel cartridge. The arrangement also includes a fuel reservoir that receives fuel from the fuel cartridge, the fuel reservoir arranged to deliver fuel to the fuel cell. The fuel reservoir also including a surface area enhanced planar vaporization membrane residing in the fuel reservoir. The combination of the surface area enhanced planar vaporization membranes residing in the fuel cartridge and reservoir provides a dual stage vaporization of fuel to the fuel cell. Other features included are passive or active arrangements to increase the temperature of the fuel or reduce pressure in the fuel container to enhance rate of vaporization.

BACKGROUND

This invention relates to powering of portable electronic devices.

Portable electronic devices are normally powered with either a primaryor a rechargeable battery. Growth in the portable electronic devicemarket, as well as, changes in usage patterns, has providedopportunities for rechargeable sources of power to power an electronicdevice. While primary batteries have a greater energy density, theirinternal resistance is larger, and primary batteries are less suitablein high drain electronic devices. Rechargeable batteries can handlelarge loads but do not have sufficient energy capacity for manyapplications.

Fuel cells incorporated into power sources for portable devices promiselonger runtimes than conventional battery systems, due to the ability touse high-energy content fuels. Several fuel cell technologies arecurrently under development for commercialization in portable powerapplications, such as direct methanol fuel cells (DMFC) and hydrogenpolymer electrolyte membrane (PEM) fuel cells.

In a DMFC, the fuel is methanol or mixtures of water and methanol.Methanol or methanol mixtures are delivered as a liquid to an anodechamber in a DMFC, where methanol is oxidized as part of theelectrochemical conversion of fuel to electricity. An operationalchallenge in DMFC systems is “methanol crossover” a phenomenon where atabove about 3% methanol concentration in the anode chamber, anunacceptably high amount of methanol migrates across a polymerelectrolyte membrane and causes both parasitic losses (reducing runtime)and mixed potentials differences at the cathode causing reduced outputpower.

Room temperature vapor phase delivery of methanol to the anode of a fuelcell has been proposed. In this approach a passive, gas permeablemembrane is placed parallel to and overlapping an anode layer in thefuel cell to convert liquid methanol to a methanol vapor at roomtemperature. The liquid methanol is provided from a fuel reservoir orfuel cartridge. The approach provides a direct path for methanoldelivery to the fuel cell system. The pure methanol vapor supplied tothe anode chamber undergoes an in situ dilution with waterback-diffusing across the PEM.

SUMMARY

Described are embodiments to enhance the rate of fuel delivery as avapor to fuel cells. An enhanced membrane is disposed in a fuelcartridge or fuel reservoir to provide fuel delivery as a vapor to fuelcells. The rate of fuel delivery is proportional to a surface area ofthe membrane. By providing compact fuel reservoir or fuel cartridgesystems vapor phase delivery of methanol fuel can be provided at higherrates to enable higher power DMFC systems.

According to an aspect of the invention, a container that supplies asource of fuel to a direct methanol fuel cell is provided. The containerincludes a housing, the housing having at least a portion of a wall ofthe housing being comprised of a thermally conductive material, a fuelegress port supported by the housing and a surface area enhanced planarvaporization membrane residing in the container.

Other embodiments are within the scope of the claims. The container hasa surface area enhanced planar vaporization membrane that is a polymermembrane. The container has the at least a portion of a wall of thehousing being comprised of a metal. The container has remaining portionsof walls of the container being thermally insulating. The container ofhas the at least a portion of a wall comprised of a thermally conductivematerial disposed adjacent the fuel egress port of the container. Thecontainer can be a fuel cartridge that contains a liquid source ofhydrogen. The liquid source of hydrogen is methanol. The fuel containeris a fuel reservoir. The fuel cartridge has at least a portion of a wallof the housing of a thermally conductive material that enhances adelivery rate of methanol in a vapor phase across the membrane todeliver vapor at the egress port of the container.

According to an additional aspect of the invention, a fuel cartridgethat supplies a source of fuel to a direct methanol fuel cell includes ahousing, the housing containing a liquid source of hydrogen and havingat least a portion of a wall of the housing being comprised of athermally conductive material and a fuel egress port supported by thehousing.

Other embodiments are within the scope of the claims. The fuel cartridgecontains methanol. The fuel cartridge has remaining portions of walls ofthe cartridge of a thermally insulating material. The fuel cartridge hasthe at least a portion of a wall of the housing being comprised of athermally conductive material is a portion of the housing of thecontainer disposed adjacent the fuel egress port of the cartridge. Theportion of the wall of the housing can be comprised of a metal.

According to an additional aspect of the invention, a method includesdisposing a fuel cartridge into a compartment of an electronic devicesuch that a portion of a wall of a housing of the fuel cartridge that iscomprised of a thermally conductive material is placed in thermalcommunication with a heat generating component in the electronic deviceto enable a vapor phase of the fuel in the housing to egress from thecartridge.

Such approaches allow the fuel cell to operate without a need for pumpsor other active controls to maintain low methanol activity in the anode.The approach also enables high rates of vapor delivery and thus permitshigher power DMFC systems than prior approaches for a specified cellsize and geometry.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are block diagrams depicting an electronic devicepowered by a fuel cell.

FIGS. 2A-2E are diagrams depicting arrangements of polymer membranes infuel cartridges.

FIG. 3 is a diagram depicting a fuel cartridge having a local heatingarrangement.

FIG. 4 is a diagram depicting a prismatic fuel cartridge having abladder and local heating arrangement.

FIG. 4A is a diagram depicting aspects of a valve for the fuelcartridge.

FIGS. 5-7 are diagrams depicting various arrangements for inducing vaporpressure differentials in fuel cartridges.

FIG. 8 is a diagram depicting a powered device and construction detailsof the fuel cartridge.

FIG. 9 is a plot of methanol vapor pressure with temperature changes.

DETAILED DESCRIPTION

Referring to FIG. 1A, a portable powered, electronic device 10(hereafter device 10) is shown. The device 10 includes a housing (notshown) having a compartment (not shown) to house an energy source, e.g.,a fuel cartridge 12. The device 10 also includes an interconnect 16 tointerface a fuel cartridge 12 that supplies a source of fuel (methanolor solutions of methanol or containing and/or carbonaceous compound ormixture of such compounds to deliver a form of hydrogen) to a fuel cell18 as a vapor rather than a liquid. The fuel cartridge 12 includes amembrane, generally denoted as 44, which partitions a liquid phase ofthe fuel to a vapor phase that can be delivered to an egress of the fuelcartridge 12 and into the fuel cell 18. Embodiments of the membrane 44are described in FIGS. 2A-2E below. Although a fuel cartridge isdescribed, other embodiments of a fuel container are included such as areservoir 13 as shown in FIG. 1B. In that instance, the fuel reservoir13 would include membrane 44 and be arranged to either receive fuel fromthe fuel cartridge 12 having the membrane 44 or be replenished withliquid fuel directly from a cartridge 12 or via a non-fixed source ofthe fuel, such as by pouring liquid fuel into the reservoir 13.

In some embodiments the fuel cell 18 is a direct methanol fuel cell(DMFC). Optionally, the interconnect 16 interfaces either a batterysource of power, e.g., primary or secondary, e.g., rechargeablebatteries (not shown) or the fuel cartridge 12. Such an interconnect 16can distinguish between a fuel cartridge and a battery and provides aconvenient technique to allow a fuel cell-powered device to operateunder battery power in situations where a fuel cartridge is temporarilyunavailable. Device 10 can be any type of portable device. Non-limitingexamples include a mobile phone, portable computer or audio/videodevice.

Referring to FIGS. 2A-2C, a fuel cartridge 12 has a fuel deliveryinterface, that is complementary to the interconnect 16 (FIG. 1),including an egress port 32, as shown. The fuel cartridge 12 includes amechanism to enhance the rate of delivery of fuel in a vapor state tofuel cells, via use of a surface area-enhanced planar vaporizationmembrane 44 residing in the fuel cartridge 12, which supplies fuel tothe direct methanol fuel cell (DMFC).

As shown in FIGS. 2A-2C, the cartridge outlet can be an egress port. Onebenefit to a narrow egress is that the cartridge 12 could be moreamenable to additional functionality, such as being easily inserted orremoved from a device without significant loss of fuel. Anotheradvantaged of a narrow opening is that optional resistive heating couldbe more finely controlled with a narrow egress (as discussed below). Apinching mechanism, for example could also be used to further restrictflow if desired.

Another approach to the egress port is as an open cavity that separatesthe cartridge 12 from the fuel cell anode (not shown). An open cavityoutlet would not disadvantageously restrict vapor diffusion to theanode, as could happen with a narrow egress. The open cavity outletcould be approximately as wide as the cartridge 12 to allow maximumtransport to the anode of the DMFC. Thus, the cartridge 12 could have atemporary cover or the like covering the opening, which is removedduring use. In some embodiments, the cartridge 12 could have a portionof the membrane 44 disposed across the opening in the cartridge 12. Ingeneral, a large opening is preferred.

The membrane 44 can be fabricated from a variety of polymer materials,including polyurethanes, silicones, poly(trimethylsilylpropyne), andothers. Fabrication of the polymer can include introducing microporosityto govern the vaporization process (via a vaporization mechanism) or adense membrane structure. The membrane can also be fabricated from asintered metal disc, coated or uncoated with polymer, to achieve asimilar vaporization performance.

Different surface area enhanced planar vaporization membranes 44 toenhance and stabilizing the rate of fuel delivery are shown in FIGS.2A-2E including a polymer membrane 46 disposed about a substantialportion of an interior perimeter of the fuel cartridge 12 to provide ahigh surface area membrane. FIG. 2B shows a composite membrane 48comprised of multiple layers or folds of polymer membrane to increasevapor permeation surface area. A membrane 50 can be arranged as a seriesof folds such as shown in FIG. 2C. FIGS. 2D and 2E show anothertechnique where a polymer membrane 52 is provided with macroscopicallyirregular (shown) or microscopically (not shown) roughened membranesurfaces to increase the effective membrane surface area forvaporization.

Referring to FIG. 2A, a gas permeable membrane 46 is shown. The gaspermeable membrane 46 spaces a liquid source of methanol 62 from a vaporphase 64 of methanol. Vapor occupies the interstitial volume between themembrane 46 and interior walls of the cartridge 12. Rather than use themembrane in planar geometry at the egress port 42, the membrane 46 ischosen to surround the fuel volume and is disposed about an interiorportion of the wall 65 of the fuel cartridge 12 enabling increasedmembrane area, and enhanced delivery rate of methanol in a vapor phaseto the egress port 42, for a given cartridge or reservoir size. The rateof fuel delivery is proportional to the surface area of the planarmembrane 46. The membrane 46 augments the rate of fuel delivery in avapor phase and can be used with regular or compact fuel reservoir orfuel cartridge systems to provide high rates of methanol fuel vapor tohigh power DMFC powered devices.

Referring to FIG. 2B, a multilayer membrane 48 includes a series oflayers 48 a or folds of polymer membrane disposed about a periphery ofthe cartridge 12 to increase membrane surface area. An example of themultilayer membrane 48 as wound-cell includes vaporization membrane 48 adisposed over a first surface of a substrate 48 b of porous materialthat holds methanol in a liquid state within pores of the material toenable the liquid methanol to migrate to the membrane 48 a and convertto a vapor phase. The membrane is fabricated from one of a variety ofpolymer systems, including polyurethanes, silicones,poly(trimethylsilyl-propyne), and other polymeric compositions,including composites. Fabrication of the polymer can include introducingmicroporosity to govern the vaporization process (via a vaporizationmechanism) or a dense membrane structure.

The membrane 48 can also be fabricated from a sintered metal disc,coated or uncoated with polymer, to achieve a similar vaporizationperformance. The substrate 48 a is comprised of one of a variety ofpolymer systems, including polyethylene, polypropylene, nylon,polyurethane, or other analogous polymers or composites of one or moreof these polymers. The substrate 48 a can also be fabricated from asintered metal form, coated or uncoated with polymer, to achieve asimilar performance.

In some embodiments the material of substrate 48 a can have furtherqualities of a “sponge-like” material. An opposite surface of the spongematerial 46 b is coated with a methanol-impermeable layer 48 c, whichcan be fabricated from materials such as a cross-linked rubber, apolymer/inorganic composite, a surface treated material such as surfacefluorinated high density polyethylene, or other methanol-impermeablematerial.

This three-layer arrangement 48 a-48 c can be wound and placed into acylindrical container that comprises the cartridge 12, with an array ofgaps between the vaporization membrane 48 a and the methanol-impermeablelayer 48 c providing a path for transporting a high flux of methanolvapor to an anode chamber in the fuel cell. This multilayer membrane 48can provide a very high flux of methanol vapor from a relatively compactfuel reservoir or fuel cartridge 12. The three-layer arrangement 48 a-48c can also be arranged as a series of planar layers and disposed inhousings of various shapes and in various configurations, such asdisposed about a periphery of the housing, at the egress port of thehousing in prismatic shaped cells as in FIG. 4 and so forth.

Various intermediate arrangements between the high surface area of awound-cell arrangement (FIG. 2B) and the rectangular,liquid-fuel-surrounding membrane (FIG. 2A) are possible. For instance,intermediately dense folded membrane 50 such as shown in FIG. 2C canbalance high fluxes obtained in the multilayer configuration and the lowmembrane volume (i.e., high fuel energy density) of option (FIG. 2A).The gas permeable membrane 50 would extend between interior walls of thefuel cartridge 12 providing a vapor chamber 51 adjacent the egress port32 of the fuel cartridge 12.

Referring to FIGS. 2D, 2E, another approach to provide a rateenhancement polymer membrane 52 is by providing a random or patternedroughening of the membrane surface (FIG. 2E). The gas permeable membrane52 is disposed between interior walls of the fuel cartridge 12 andprovides a vapor chamber 51 adjacent the egress port 32 of the fuelcartridge 12. The roughening can be on one or both sides of themembrane. One side of the membrane (commonly the vapor side) may limitthe permeation rate. It is preferable to enhance thepermeation-rate-limiting side of the membrane.

While room temperature vapor phase delivery of methanol to the anode ofa fuel cell using a passive, a gas permeable membrane placed parallel toand overlapping the anode layer in the fuel cell can work well for lowpower (<3 W) DMFC systems) such an approach may not provide sufficientmethanol vapor flux to sustain higher power operation. This is due tofundamental limitations in the membrane-enabled vaporization process.The flux of methanol per unit area of membrane is sufficient to maintainoxidation of methanol at reasonable rates for a similar area of theanode. However, above a power range of several Watts, the area of themembrane needs to grow unreasonably large to maintain the methanol fluxneeded to sustain fuel cell operation at higher power. A fuel cartridgewith the geometric dimensions needed to provide the flat membrane areafor higher power operation is not convenient for consumer use. Inaddition, large membranes can be mechanically unstable and have a higherlikelihood of mechanical failure over time. Dependent upon operatingpoint and choice of membrane material, an example power range of, e.g.,1 W could require a membrane area of 0.7 cm², whereas a 5 wattapplication could require a membrane area of 3.3 cm². At 3.3 cm² andhigher this becomes impractical for many consumer applications becauseit requires a very large membrane surface area.

Localized heating can be used in conjunction with the above approaches,either via a resistive element that is disposed in the cartridge or byuse of heat generated from the electronic device.

The approaches described above result in an augmentation of theeffective surface area of the membrane arrangement generally 44 (andthus an overall rate of vapor permeation) over a fixed geometric area.An enhanced membrane 44 disposed in a fuel cartridge or fuel reservoirprovides fuel delivery as a vapor to fuel cells at a rate proportionalto the enhanced surface area of the membrane. The enhanced surface areamembrane permits compact fuel reservoir or fuel cartridge systems thatcan deliver a vapor phase of methanol fuel at higher rates to enablehigher power DMFC systems. Such an approach also allows the fuel cell tooperate without a need for pumps or other active controls to maintainlow methanol activity in the anode.

Referring to FIG. 3, a resistive heating element 72 is disposed at avaporization membrane interface 44 to enhance vapor fuel delivery, asshown. The rate of vaporization increases significantly with increasesin temperature. The vaporization membrane arrangements described inFIGS. 2A-2D can use the heating element 72 as a localized heat source toincrease temperature and hence rate of vaporization. The heating element72, illustrated in FIG. 3 is disposed electrically in parallel with theprimary load (device 10) and is powered by a small fraction of the fuelcell electrical output to provide a net boost in output power.

One example of the heating element 72 is a wire, e.g., a coiled wirehaving a relatively high resistivity characteristic. A typicalresistivity characteristic for the heating element 72 as a wire is in arange of 10 to 1 M ohms/cm. The heating element 72 can be comprised of arelatively high resistivity material such as Tungsten. Other materialsthat can be used include nickel/chrome alloys and others. The highresistivity materials can be coated with a polymer or a precious metalto provide protection against erosion and contamination of the fuelcell. The resistive element 72 is disposed in thermal communication withone of the vaporization membrane 44 arrangements (e.g., any of theembodiments in FIGS. 2A-2E, or other configurations).

The membrane 44 and resistive element 72 provide a vapor chamber 74,e.g., a space between the liquid fuel 76 with or without the egress port32 of the cartridge 12 principally occupied by a vapor phase of thefuel. Preferably, the resistive heating element 72 directly contacts themembrane 44, since as the membrane temperature increase that augmentsthe vaporization rate. The heating element 72 could be on the liquidside or on the vapor side of the membrane 44, or embedded within themembrane 44. The latter two options (vapor side and embedded) providethe advantage of minimizing unnecessary heating of the liquid in thecartridge. Additionally, a sintered metal, for example, could serve asboth the membrane material and resistive heater. Heat provided by theresistive element 72 enhances the rate of vaporization across themembrane 44 and can improved overall performance when the device 10powered by the fuel cell is used in relatively cold ambient temperatureenvironments.

Referring to FIG. 4, another approach 80 can vaporize the liquid fuel,e.g., methanol in a fuel cartridge 12 entirely through a thermal processwithout the need for a membrane. In this arrangement, power is drawnfrom the fuel cell (not shown), or supplied through a small battery 82(button cell, for example) located within or on the fuel cartridge 12 topower a heating mechanism 84. Here, the heating mechanism 84 isschematically shown without connections to the battery, as a wiredisposed at the egress port 32 of the fuel cartridge 12.

The fuel cartridge 12 includes a wall or body, here illustrated as aprismatic battery case 86 including the heating element 84, and aninternal fuel bladder 90 of a fuel impermeable material, e.g., a rubberand the like that is in contact with a movable wall or piston 88 in theinterior of the fuel cartridge 12. A spring 89 applies force to thewall. Guides (not shown) can be used to guide the wall or piston 88 asit moves along the length of the prismatic case. Liquid fuel, e.g.,methanol is disposed in the bladder 90. As liquid is consumed from thefuel cartridge 12 the pressure in the bladder 90 subsides, allowing theforce produced by the spring 89 to urge the wall or piston 88 againstthe bladder 90 to insure that methanol in the bladder 90 is delivered tothe egress port 32 of the fuel cartridge 12. The wall/piston 88 andspring 89 insure uniform delivery of liquid from the bladder 90independent of case orientation.

The egress port 32 can have a fuel valve integrated with a vaporizationheating unit. One embodiment as shown in FIG. 4A, includes a resistiveheating element(s) that is disposed in a constricted area within thevalve assembly (not shown). In some embodiments the heat element 84could be dispensed with. Power for the resistive heaters can be obtainedby the button cell battery within or supported on the fuel cartridge, orfrom the fuel cell power source via external leads (not shown). Otherembodiments are possible.

Referring to FIG. 4A, an example of a fuel valve 70 having an integratedvaporization-heating unit is shown. The fuel valve 70 is illustrated asthe egress 32 for the embodiment of the cartridge 12 shown in FIG. 4including membrane arrangement 46. The egress 32 is depicted as a valve33 having an integrated heating element 73. The valve 33 is supported onthe cartridge wall 65 and includes the heating element 73 arranged inany one of a variety of configurations such as disposed in the center ofthe valve as shown, or disposed about the sidewalls of the valve (notshown) or integrated into the sidewalls (not shown). The heating elementis disposed to increase the rate of vaporization across the membrane 46.The valve can have various mechanisms to secure it to a device duringuse, such as a bayonet connection, threaded connection and so forth.

Referring to FIG. 5, an alternative arrangement to enhance vapordelivery is to provide a reduced pressure on the permeate (vapor) sideof a vaporization membrane 44 and take advantage of the principle that apressure decrease (similar to a temperature increase) can boil orevaporate a liquid. Stated differently, a reduced pressure downstreamincrementally decreases a vapor concentration of fuel, thus increasing adriving force for permeation of the fuel from the liquid phase to thevapor phase.

One mechanism to induce a reduced pressure is to increase volume on avapor side 90 of the cartridge 12. The vapor side of the cartridge 12includes a vapor permeable piston 92 that is urged against liquid 96 inthe cartridge 12 by one or more spring mechanisms 94 disposed betweenthe piston 92 and interior regions of the cartridge 12 adjacent theegress port 32 of the cartridge 12. One embodiment of the piston 92 isas a vaporization membrane 44. A wire mesh or rigid micro- ormacro-porous layer can mechanically support a flexible vaporizationlayer, (e.g., a fluorocarbon polymer, polyethylene, polypropylene,polycarbonate, polyimide, polysulfone, polysulfide, polyurethane,polyester, cellulose, or paper). The ring piston 92 provides aleak-proof seal while sliding along the cartridge wall. The ring outerdiameter nests barely within the cartridge diameter. Also, the ring andadjacent cartridge wall are preferably made of or coated by a fuelrepellent and fuel impermeable material to minimize liquid flow leakageinto the vapor side. Such a materials or coatings are fluoropolymers,e.g., polytetrafluoroethylene and so forth. In addition for the ring inparticular, a sufficiently rigid material is preferred to minimize thering radial thickness while still providing mechanical stability,allowing for maximum uncovered membrane area.

As the liquid volume is depleted, the vapor side increases in volumesince the piston 92 travels further away from the egress port 32expanding the volume on the vapor side of the cartridge 12. Again, thevaporization membrane 44 contains the fuel in its liquid phase andprincipally allows only vapor to permeate into the vapor side 90. Themechanical action can be active (e.g. with the force of springs) orpassive (e.g., with liquid displacement alone). Passive actuation relieson low friction of the ring piston.

Referring to FIG. 6, fuel cell 18 is shown as a fuel cell stack 100 (asingle membrane electrode assembly) having an anode 102 and a cathode103 spaced by a separator 105. The fuel stack 100 is disposed adjacentthe vapor side 90 of the fuel cartridge 12. Vapor from the fuelcartridge 12 directly flows to an anode electrode 102 of the fuel cell18.

The volume of expansion induced in the vapor side 90 of the cartridge 18can be made greater than the contraction volume of the liquid fuel phaseby permitting additional expansion of the volume of the vapor chamber74.

Referring to FIG. 7, an arrangement 110 to enhance vapor delivery byproviding additional volume to the vapor phase chamber 74 is shown. Thevapor side 90 of the fuel cartridge 12 including piston 92 and internalspring 94, as in FIG. 5, is augmented with an arrangement 110 toincrease the effective volume of the vapor chamber 74 of the cartridge12. Additional volume is provided to the vapor phase chamber 74 by anexternal chamber 112 that is disposed around the outer surface of thecartridge 12 and which is in vapor communication with the internal vaporchamber 74. The external chamber 112 has a vapor impermeable piston 114that is urged against vapor in the outer chamber 112 in the cartridge 12by one or more outer spring mechanisms 116 disposed between the vaporimpermeable piston 114 and the fuel cell 18, adjacent the egress port 32of the cartridge 12. As the vapor pressure increases, the increase invapor pressure causes the piston 114 to move in a manner that increasesthe volume of the external chamber 114.

One embodiment of the vapor impermeable piston 114 is a solid sealingmaterial or metal coated with sealing material such aspolyfluoroalkenes, fluoroelastomers, and rubbers, e.g., silicone,fluorosilicone, nitrile neoprene, natural, or polyurethane. A metal corecan be included in the ring piston to provide mechanical rigidity. Theexternal chamber 114 may be an expandable gas volume of fuel vapor,anode reaction product, and possibly inert gas (such as nitrogen). Thecontracting volume opposing the external chamber 114 (i.e., on theopposite side of the ring piston) is preferably vented to an externalambient to avoid pressure buildup inside the external chamber 114.

The expansion may be independent of liquid depletion as shown here withindependent springs. Alternatively, the outer ring piston may beconnected mechanically (or magnetically if desired) to slide in parallelwith the inner piston movement with liquid depletion. Furthermore, thevapor side cavity may be shaped (e.g., cone-like) to allow for anincreasing volume expansion as the fuel depletes. Vapor-side expansionsgreater than the liquid contraction do have the disadvantage ofrequiring additional overall volume.

For control of fuel delivery, the membrane may be synthesized orprocessed (by localized compression or elongation, for example) to havevariable permeability with surface position. For instance, if anon-uniform distribution of fuel to the anode is provided, aposition-variable permeability (and thus variable fuel flux) can beprovided to even fuel distribution.

Referring to FIG. 8, the portable powered, electronic device 10 depictedin FIGS. 1A and 1B, is shown with a housing 11, having a compartment 14that houses an energy source, e.g., one of the fuel cartridges 12 asdescribed above. The interconnect 16 interfaces the fuel cartridge 12that supplies a source of fuel (a form of hydrogen) to the fuel cell(not shown) as a vapor rather than a liquid. The fuel cartridge 12includes vaporization membrane 44 that partitions a liquid phase of thefuel to a vapor phase that can be delivered to an egress 32 of the fuelcartridge 12. In some embodiments of the fuel cartridge 12 the walls orat least portions of a wall, e.g., 12 a of the fuel cartridge 12 arefabricated from a thermally conductive material, typically a metal. Suchan embodiment of a fuel cartridge 12 uses the walls of the fuelcartridge as a heat sink for heat generated by small portable deviceslike a lap top computers. The metal or conductive material or at leastthose portions of the cartridge comprised of the conductive material aredisposed in thermal communication with a heat-dissipating component 19within the device 10. The fuel cartridge is disposed in close proximityto heat dissipating component 19, e.g., a CPU in a laptop, or within anairflow pattern associated with micro fans (not shown) used in someportable power devices.

The fuel cartridge 12 draws heat away from heat dissipating component 19in the electric device 10. Heat will be transferred across the thermallyconductive wall of the fuel cartridge 12 and will provide a concomitantincrease in the pressure of methanol vapor within the cartridge 12. Theincrease in vapor pressure enables faster vapor flow through theseparator membrane 44. This technique provides a fuel cartridge 12 witha passive system that provides enhanced methanol vapor pressure andhence greater energy delivery to the fuel cell. In addition, the use ofthe fuel cartridge 12 as a heat sink may significantly reduce the needfor a cooling fan (also an energy drain on the device) to enhance deviceefficiency and increase run time of the device. The exact configurationof the fuel cartridge 12 could be dependent on the configuration of thedevice 10, the amount of heat generated by the device and the presenceor absence of a fan.

Configurations of the fuel cartridge 12 can include, a metal or otherthermally conductive material wall 12 a that is combined with remaining,thermally insulating walls 12 b of the fuel cartridge 12 b. Thethermally conductive walls 12 a would be disposed in direct contact withthe heat source 19 in the device or at least in close proximity to theheat source 19, or in an air flow path (not shown) that is used toremove heat from the heat source 19. Alternatively, the thermallyconductive can be an upper portion of the fuel cartridge 12 adjacent thefuel egress port 32 and in general alignment with the vapor chamberprovided in the cartridge. In some embodiments, the housing of the fuelcartridge 12 can be completely comprised of metal or other thermallyconductive material. The fuel cartridge can take various shapesincluding the prismatic type depicted, cylindrical types depicted inFIGS. 1, 2A-2D and so forth.

Referring to FIG. 9, a plot that depicts changes in methanol vaporpressure with temperature changes is shown.

The cartridge 12 is particularly useful with electrical components thatgenerate a large amount of heat during operation. The cartridge 12 wouldhave features that take advantage of heat generating surfaces in thedevice ideally being placed in direct contact with the fuel cartridge.In some embodiments, the cartridge can be configured as a fuel reservoirand supplement or replace heat sink elements on heat dissipatingdevices. The cartridge containing the methanol liquid serves as a vaporphase fuel delivery system and a heat sink for the device 10. Thus, thefuel cartridge acting as a heat sink helps to remove heat from thedevice 10, while the heat generated increases the vapor pressure of themethanol vapor and therefore increases the amount of vaporized fuel thatcan be delivered by the membrane surface to the fuel cell. The fuelcartridge can include external and/or internal fins to increase heattransfer to the methanol fuel.

In pervaporation, the fuel is vaporized as it moves through themembrane, rather than being vaporized in advance of the membrane. Someembodiments of the membrane can be considered pervaporation membraneswhereas; others can be considered vaporization membranes. For instance,direct heating without a membrane or in advance of the membrane(vapor-vapor permeation) is a direct vaporization process.

The approaches described above in FIGS. 2A-2E result in an augmentationof the effective surface area of the membrane arrangement generally 44(and thus an overall rate of vapor permeation) over a fixed geometricarea. An enhanced membrane 44 disposed in a fuel cartridge or fuelreservoir provides fuel delivery as a vapor to fuel cells at a rateproportional to the enhanced surface area of the membrane. Thearrangements depicted in FIGS. 3-9 increase vaporization rateexponentially with increases in the temperature of the liquid fuelsource. The enhanced surface area membrane and/or heating or pressurereducing mechanisms permit compact fuel reservoir or fuel cartridgesystems that can deliver vapor phase of methanol fuel at higher rates toenable higher power DMFC systems. Such an approach also allows the fuelcell to operate without a need for pumps or other active controls tomaintain low methanol activity in the anode.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, rather than being a replaceable fuel cartridge, the arrangementcan be a permanently attached fuel reservoir that can be replenishedperiodically through a refilling mechanism. In addition, a fuelcartridge could be used to provide vapor phase methanol fuel to a fuelcell assembly that has a permanently attached fuel reservoir containinga second membrane system. In such a system, the second membraneregulates the flux of vapor phase methanol to the fuel cell in two-stagea manner that may provide more control of vapor delivery than that of asingle-stage vaporization approach. The techniques thus apply to a fuelcell assembly with a permanently attached fuel reservoir, or replaceablefuel cell cartridge, or both. Accordingly, other embodiments are withinthe scope of the following claims.

1. A container that supplies a source of fuel to a direct methanol fuelcell, the container comprising: a housing, the housing having at least aportion of a wall of the housing being comprised of a thermallyconductive material; a fuel egress port supported by the housing; and asurface area enhanced planar vaporization membrane residing in thecontainer.
 2. The container of claim 1 wherein the surface area enhancedplanar vaporization membrane is a polymer membrane.
 3. The container ofclaim 1 wherein the at least a portion of a wall of the housing beingcomprised of a thermally conductive material is comprised of a metal. 4.The container of claim 1 wherein remaining portions of walls of thecontainer are thermally insulating.
 5. The container of claim 1 whereinthe at least a portion of a wall of the housing being comprised of athermally conductive material is a portion of the housing of thecontainer disposed adjacent the fuel egress port of the container. 6.The container of claim 1 wherein the container is a fuel cartridge. 7.The container of claim 1 wherein the cartridge contains a liquid sourceof hydrogen.
 8. The fuel cartridge of claim 1 wherein the liquid sourceof hydrogen is methanol.
 9. The fuel cartridge of claim 1 whereincontainer is a fuel reservoir.
 10. The fuel cartridge of claim 1 whereinat least a portion of a wall of the housing being comprised of athermally conductive material enhances a delivery rate of methanol in avapor phase across the membrane to deliver vapor at the egress port ofthe container.
 11. A fuel cartridge that supplies a source of fuel to adirect methanol fuel cell, the fuel cartridge comprising: a housing, thehousing containing a liquid source of hydrogen and having at least aportion of a wall of the housing being comprised of a thermallyconductive material; a fuel egress port supported by the housing. 12.The fuel cartridge of claim 11 wherein the liquid is methanol.
 13. Thefuel cartridge of claim 11 wherein remaining portions of walls of thecartridge are thermally insulating.
 14. The fuel cartridge of claim 11wherein the at least a portion of a wall of the housing being comprisedof a thermally conductive material is a portion of the housing of thecontainer disposed adjacent the fuel egress port of the cartridge. 15.The fuel cartridge of claim 11 wherein the at least a portion of a wallof the housing being comprised of a thermally conductive material iscomprised of a metal.
 16. A method comprises: disposing a fuel cartridgeinto a compartment of an electronic device such that a portion of a wallof a housing of the fuel cartridge that is comprised of a thermallyconductive material is placed in thermal communication with a heatgenerating component in the electronic device to enable a vapor phase ofthe fuel in the housing to egress from the cartridge.
 17. The method ofclaim 16 wherein fuel cartridge contains a source of an oxidizable fuel.